The development of the production technology of coated paper is remarkable. Hitherto, the principal object of the technology has been rather to develop the coating step. However, in recent years, following an increase in the coating rate, an attention has also been paid to the drying step from the point of view of needs of an increase in the drying rate as well as of improvements in the quality, and IR dryers, flotation dryers and so on, have been developed.
Coated papers are produced by coating paper with coating solution comprising substances or chemicals for the purposes of imparting new physical properties or functions and then drying it. Typical examples of them include clay-coated paper, a pressure-sensitive recording paper, and a thermo-sensitive recording paper. The coating as referred to herein includes not only mere surface coating of paper but also impregnation of chemical solution into the paper layer. In the drying method which has hitherto been used for these coated papers, heated air is used as a drying heat source and brought into direct contact with paper sheet in a drying chamber, and the heated air provided for drying is discharged out from the drying chamber as waste gas together with water vapor evaporated from the paper sheet. However, in this method, since the heated air has no condensation latent heat, its heat capacity is relatively small so that a considerable period of time is required for warming up the paper to the adiabatic saturation temperature. Although the heated air provided for drying contains the evaporated water vapor and has high heat energy, it is limited in terms of the recovery and circulation for re-use and hence, is discarded in many cases. For this reason, the heat efficiency in this drying method is low.
Drying of a substance by using superheated steam has already been employed in the mining and food fields, and also, the drying paper by using superheated steam is not a novel means in the paper manufacture field. So far, it has been reported in the literatures to apply this drying method for drying a wet hand-sheet made up by using a hand sheet machine and to compare with the usual drying. However, no practical method of drying by using superheated steam has been disclosed. In particular, there is no report directed to the application for coated papers. The reasons for this are that the qualities of paper produced by using high-temperature superheated steam and advantages of the heat efficiency have not been clarified yet and that the proportion of the air to the steam in the heating medium has been liable to change. From the standpoint of practical use, there is a fear about instabilization in the product quality and productivity.
In recent years, following an increase in the coating rate owing to the improvement in-productivity, there has been a problem in shortage of the drying ability. However, for the conventional drying method of coated paper, while it is possible to improve the drying ability by increasing the adiabatic saturation temperature, there is still a great problem in terms of the heat efficiency as described above. On the other hand, because of an innovation in the printing technology as well as of a development in OA instruments, in particular, an increase in the speed and an improvement in the image quality of printing machines and printers,the present products of these coated papers are not always satisfactory in terms of mottling at the time of color printing on a coated printing paper (color shading at the time of multi-color printing), stiffness, dimensional stability (including anti-curl properties), ink absorbency, and so on.
From these standpoints of view, the present inventors have made extensive investigations about a quite new process for drying coated paper, which is free from the conventional concept, and attempted to improve the heat efficiency in the drying as well as the quality of coated paper, both of which have been considered to be of problem.